Exposure to ionizing radiation (such as X-rays, gamma rays, and alpha- or beta-radiation) can cause damage to cells. This damage can result in cell death (e.g., through apoptosis), or can cause genetic changes in the cell, resulting in unchecked cell proliferation and cancer.
While, in general, exposure to such radiation is therefore undesirable, the administration of carefully monitored doses of radiation is an accepted treatment for certain cancers; by targeting the radiation to a tumor, cancer cells can be destroyed. A frequent complication of radiotherapy is the irradiation of normal tissues surrounding the cancerous tissues. Such normal tissues are often damaged by the radiation, resulting in undesired radiation injury to normal cells and tissues, which can have severe consequences for the affected patient.
Exposure to radiation can occur in several other ways, including exposure to normal background levels of radiation (such as cosmic rays or radiation due to naturally-occurring isotopes present in the earth) or elevated environmental radiation (including occupational exposure of workers in medical facilities or nuclear power plants). Another potential source of exposure to certain types of radiation is the accidental or intentional release of radioactive materials, for example, as the result of an accident or as a result of terrorist activity, e.g., as the result of a radiologic weapon such as a so-called “dirty bomb” (an explosive device intended to spread radioactive materials so as to contaminate an area).
The primary form of protection against radiation injury is avoidance of exposure to radiation. For example, shielding materials capable of preventing penetration of radiation into the body can be used when a source of radiation is known; e.g., lead aprons can be used to block x-rays. Protective clothing can be used to prevent contamination of the body with radioactive materials, and decontamination procedures can be used to remove radioactive materials.
Prophylactic treatment with iodine can be used to prevent accumulation of radioactive iodine in the thyroid gland, and thus to prevent radiation injury to the thyroid, but such treatment does not provide protection against other radioactive elements or other organ sites and cannot prevent injury once the radioactive isotope has delivered a dose of radiation to the tissue. Treatment with radioprotectants such as amifostine (an approved radioprotectant) is effective in preventing certain types of radiation damage, such as DNA damage due to free radicals (or other reactive species) produced by the radiation. However, the approved indications for amifostine are limited, and side-effects such as nausea have been noted.
Another compound, 5-androstenediol, has been tested as a radiation protectant in preclinical animal studies. This compound is reported to improve survival in mice exposed to radiation, possibly by stimulating production of neutrophils and other immune-system cells and thus preventing infection, a significant cause of death in radiation-injured subjects. However, this compound is a salvaging measure and it does not counteract the pathogenic mechanism of radiation nor protect organs other than the hematopoietic system. It has not yet been approved for human use.
While radiation injury and radiation sickness can be treated symptomatically, in most cases, it is difficult to prevent or ameliorate radiation damage or injury to cells once the exposure to radiation has occurred.
β-lapachone (3,4-dihydro-2,2-dimethyl-2H-naphtho[1,2-b]pyran-5,6-dione) is a simple non-water soluble orthonapthoquinone which can be isolated from the heartwood of the lapacho tree (See Hooker, S. C., (1936) J. Am. Chem. Soc. 58:1181-1190; Goncalves de Lima, O., et al., (1962) Rev. Inst. Antibiot. Univ. Recife. 4:3-17). The structure of β-lapachone was established by Hooker in 1896 and it was synthesized by Fieser in 1927 (Hooker, S. C., (1936) J. Am. Chem. Soc. 58:1181-1190).
β-lapachone has been shown to have a variety of pharmacological effects. Numerous derivatives have been synthesized and tested as anti-viral and anti-parasitic agents, and β-lapachone has been shown to have anti-trypanosomal effects (See Goncalves, A M et al. (1980) Mol. Biochem. Parasitology 1:167-176; Schaffner-Sabba, K. et al. (1984) J. Med. Chem. 27:990-994; Li, C J et al., (1993) Proc. Natl. Acad. Sci. USA 90:1839-1842). β-lapachone significantly prolongs the survival of mice infected with Rauscher leukemia virus, probably through inhibition of reverse transcriptase (Schaffner-Sabba, K. et al. (1984) J. Med. Chem. 27:990-994; Schuerch, A R et al., (1978 Eur. J. Biochem. 84:197-205). β-lapachone inhibits viral replication and gene expression directed by the long terminal repeat (LTR) of the human immunodeficiency virus type I (Li, C J et al., (1993) Proc. Natl. Acad. Sci. USA 90:1839-1842).
There have been reports that β-lapachone induces cell death in human prostate cancer cells (See Li, C J et al., 1 (1995) Cancer Res. 55:3712-3715) and that β-lapachone induces necrosis in human breast cancer cells, and apoptosis in ovary, colon, and pancreatic cancer cells mediated by caspase activation (Li, Y Z et al., (1999) Molecular Medicine 5:232-239). In addition, β-lapachone, when combined with paclitaxel (sold under the name Taxol® by Bristol-Myers Squibb Co., N.Y., N.Y.) at moderate doses, has effective anti-tumor activity in human ovarian, prostate and breast cancer xenograft models in nude mice (See Li, C J et al. (1999) Proc. Natl. Acad. Sci. USA 96:13369-13374).
β-lapachone was investigated as a sensitizer of cancer cells to ionizing radiation and DNA damaging agents (Boorstein, R J et al., (1984) Biochem Biophys. Res. Commun. 118:828-834; Boothman, et al., (1989) Cancer Res. 49:605-612). The combination of β-lapachone administration with X-ray irradiation was found to result in synergistic (increased) cytotoxicity in vitro in a human radioresistant malignant melanoma cell line; the authors of this report (Boothman, et al., (1989) Cancer Res. 49:605-612) suggest that β-lapachone inhibits the ability of the malignant cells to repair potential lethal DNA damage caused by the radiation.
The present inventors have now unexpectedly discovered that administration of a modulator of cell cycle checkpoint activation (e.g., an activator of a cell cycle checkpoint or checkpoints, such as β-lapachone) is effective for the protection of normal (e.g., non-cancerous) cells and organisms against radiation damage or injury, or the treatment of radiation damage or injury in normal cells and organisms, or both.